In aviation, the total air temperature (TAT) is measured by a temperature sensor mounted on the surface of the aircraft. For example, FIG. 1 shows an aircraft 10 with an engine 11 mounted to a fuselage or wing 12. The nacelle 13 of the engine 11 includes a front end 14 for receiving air flow and an aft end 15 for exhausting air flow and combustion products. The nacelle 13 also includes an inner wall 16 disposed in front of the fan 17. A sensor assembly 20 is mounted on the inner wall 14. The sensor assembly 20 is designed to bring the air entering the engine 11 to rest relative to the aircraft 10. As the air is brought to rest, kinetic energy in the air is converted to internal energy. The air is compressed and experiences an adiabatic increase in temperature. Therefore, the TAT is higher than the static (or ambient) air temperature. However, the static air temperature is obtained from the TAT and the true airspeed of the aircraft 10 is calculated from the static air temperature. Thus, the TAT is an essential input to an air data computer in order to enable computation of the static air temperature and the true airspeed.
Conventional TAT sensors do not work properly in icing conditions. During flight in icing conditions, water droplets, and/or ice crystals, engage the sensor and can build up or accrete around the opening of the internal sensing element. An ice ridge can grow and eventually break free—clogging the sensor temporarily and causing an error in the TAT reading. To address this problem, some conventional TAT sensors have incorporated an elbow, or bend, to inertially separate these particles from the airflow before they reach the sensing element.
Other designs that enhance the anti-icing performance of the TAT sensors include heating elements embedded in or around the housing walls of TAT sensors or from hot air flowing through the sensors (e.g., from an aircraft engine). See, e.g., US 2011/0106475. Unfortunately, use of external heating also results in heating of the internal boundary layers of the air which, if not properly controlled, adversely affect the accuracy of TAT measurement. In short, the heat used to de-ice TAT sensors causes errors in the temperature reading, which are difficult to correct for.
Still other designs attempt to avoid the use of a heating element by employing a protective airfoil in front of the sensor and shields around the sensor as shown in U.S. Pat. No. 7,845,222 and depicted in FIG. 2. The prior art sensor 21 of the sensor assembly 20 is shielded from the incoming water droplets by the protective airfoil 22 and the shields 23, 24, wherein the airfoil 22 is disposed forward of the shields 23, 24 and sensor 21. This shielding prevents ice accretion on the sensor 21. However, placement of the airfoil 22 in front of the sensor 21 can cause the sensor 21 to be disposed in a wake or eddy region, leading to inaccurate measurements caused by recovery errors, which are also difficult to correct for.
Therefore, improved TAT sensor assemblies that accurately measure the TAT during icing conditions are still needed.